Methods of assessing contact between an electrode and tissue using complex impedance measurements

ABSTRACT

A system and method measures impedance across a plurality of electrodes and assesses proximity or contact between electrodes of a medical device and patient tissue. Contact is assessed between individual electrodes and cardiac tissue using bipolar electrode complex impedance measurements. Initially, baseline impedance values are established for each of the individual electrodes based on the responses of the electrodes to the applied drive signals. After establishing the baseline impedance values a series of subsequent impedance values are measured for each electrode. For each electrode, each subsequent impedance value may be compared to a previous baseline impedance value for that electrode. If a subsequent impedance value is less than the baseline impedance value for a given electrode, the baseline impedance value may be reset to the subsequent impedance value. Such systems and method are particularly applicable to medical devices having numerous electrodes.

CROSS REFERENCE

The present application claims the benefit of the filing date of U.S.Provisional Application No. 62/607,554 having a filing date of Dec. 19,2017, the entire contents of which is incorporated herein by reference.

BACKGROUND a. Technical Field

The instant disclosure relates to electrical impedance-based measurementof electrodes of a medical device to determine, among other things,contact between tissue and the electrodes of the medical device. Morespecifically, the disclosure relates to simultaneously sensing theproximity of multiple electrodes to tissue in a body

b. Background Art

Catheters are used for an ever-growing number of procedures. Forexample, catheters are used for diagnostic, therapeutic, and ablativeprocedures, to name just a few examples. Typically, the catheter ismanipulated through the patient's vasculature and to the intended sitesuch as, for example, a site within the patient's heart. The cathetertypically carries one or more electrodes, which may be used forablation, diagnosis, and the like.

In many procedures, it may be beneficial to know the contact status ofan electrode (e.g., in contact with tissue, in a blood pool) on acatheter. For example, in an electrophysiology mapping procedure, theelectrical signal present on an electrode may vary depending on whetherthe electrode is in contact with tissue, or adjacent to the tissue in ablood pool, and that difference may be accounted for in software. Inanother example, in an ablation procedure, it may be desirable to onlydrive an ablation current when an electrode is in contact with thetissue to be ablated.

One existing methodology that may be used to determine whether anelectrode on a catheter is in contact with tissue includes driving acurrent between the electrode and an electrode elsewhere within thepatient (e.g., at a stable position within the patient) or on theexterior of the patient (e.g., on the patient's skin) and assessing theimpedance between the electrodes. To determine an impedance betweenthose electrodes, the electric potential of the electrode on the medicaldevice may be referenced to a third electrode, which may also beelsewhere within the patient or on the exterior of the patient.

The foregoing discussion is intended only to illustrate the presentfield and should not be taken as a disavowal of claim scope.

BRIEF SUMMARY

Measuring the impedance of an electrode has been demonstrated to providea reliable method of detecting when an electrode comes in contact withtissue (e.g., intracardiac tissue). Specifically, due to the reducedconductivity of tissue compared to blood, the impedance of an electrodeis significantly higher once it comes in contact with the tissue thanwhen the electrode is disposed within a blood pool (e.g., internalpatient cavity). The present disclosure is directed to assessing contactbetween an electrode and tissue using impedance measurements. In oneembodiment, the disclosure is directed to assessing contact between anelectrode and cardiac tissue using bipolar electrode complex impedancemeasurements. Such assessment may be implemented in a system, such as anelectronic control unit, which measures impedances between electrodes ofa connected medical device. Such a system may include a controller orfrequency source configured to generate a plurality of drive signals.Each of the drive signals may have a unique modulation frequency thatmay be a harmonic of a common base frequency. The controller orfrequency source may further be configured to simultaneously apply eachof the plurality of drive signals across an individual pair ofelectrodes of the connected medical device. The medical device may be acatheter. However, the system is not limited to use with catheters andmay be utilized with other medical devices. The system may include ameasurement circuit for measuring responses of the drive signals asapplied to individual pairs of electrodes of the medical device. Themeasurement circuit may include a demodulator that is configured tosimultaneously demodulate the response signal(s) for each unique drivefrequency. The demodulator may generate demodulation signals each havingan identical frequency to one unique frequency of the drive signal andknown phase that is different than a phase of the unique frequency. Suchdemodulation may include quadrature demodulation to provide in-phase andquadrature channels. Additional hardware and/or software may scale theresults to resistive and reactive impedance in units of ohms.

In one arrangement, a system and method are provided for establishingbaseline impedance values for electrodes of a medical device such thatsubsequent impedance changes of those electrodes may be utilized toassess tissue contact and/or when an electrode enters and exits anintroducer (i.e., is sheathed or unsheathed). The system and methodinclude simultaneously applying a plurality of drive signals havingunique frequencies across different individual pairs of electrodes of amedical device. Initially, a baseline impedance value is measured foreach of the plurality of electrodes based on the responses of theelectrodes to the applied drive signals. Measuring the impedance valuesmay further include synchronously demodulating responses of theelectrodes to the simultaneously applied drive signals. The applicationof the drive signals and measuring of impedance values may continue overa predetermined period of time. Accordingly, after the initial impedancemeasurements, a series of subsequent impedance values may be measuredfor each electrode. For each electrode, each subsequent impedance valuemay be compared to a previous baseline impedance value for thatelectrode. If a subsequent impedance value is less than the baselineimpedance value for a given electrode, the baseline impedance value maybe reset to the subsequent impedance value. In this regard, the lowestmeasured impedance value may be established as a baseline impedancevalue for a given electrode.

The system and method may each be utilized for medical devices having ahigh number of electrodes. In such applications, a medical device may bemoved relative to an internal patient cavity in conjunction withmeasuring the impedance values. Such movement allows the electrodes tomove into and out of contact with patient tissue. The near continuousmeasurement of impedance values in conjunction with the movement of themedical device allows for determining baseline impedance values whilethe electrodes of the medical device are free of contact with patienttissue.

The system and method may further include comparing subsequent impedancevalues to an establish baseline impedance values to generate anindication of tissue proximity between an electrode and patient tissue.For example, if a subsequent impedance value is greater than a baselineimpedance value, the change between the subsequent impedance value andthe baseline impedance value may be assessed to determine tissueproximity. Such an indication of tissue proximity may include a binaryindication of contact and noncontact between an electrode and tissue.Alternatively, the indication of tissue proximity may provide a range ofcontact conditions between the electrode and the tissue. Further, theestablished baseline impedance values may be utilized to determine whenan electrode is sheathed or unsheathed.

The system and method may further include displaying indications oftissue proximity for each electrode on a display device. In addition,such displaying may include identifying a location of each electroderelative to an internal patient cavity such that the indication oftissue proximity may be displayed at a corresponding location of a mapof the internal patient cavity.

In another arrangement, a system and method are provided for dynamicallyestablishing baseline impedance values for electrodes of a medicaldevice during a medical procedure and generating an indication of tissueproximity between the electrodes and patient tissue. The system andmethod include applying a plurality of drive signals, each having aunique frequency, across different individual pairs of electrodes of amedical device. The drive signals may be applied in conjunction with theapplication of, for example, ablation energy to one or more of theelectrodes. A series of impedance values may be measured for each of theplurality of electrodes in response the applied drive signals. Each ofthe series of impedance values may be compared to a prior baselineimpedance value for the electrode. If the subsequent impedance value isgreater than the baseline impedance value, an indication of tissueproximity may be generated and displayed on a display device. This mayentail identifying a location of the electrode relative to an internalpatient cavity, wherein the indication of tissue proximity is displayedon the display device at a corresponding location of a map of theinternal patient cavity. If a subsequent impedance value is lower thanthe baseline impedance value, the baseline impedance value may be resetto the subsequent impedance value.

In either of the noted systems or methods, the impedance values maycomprise complex impedance values having an in-phase (e.g., real)component and a quadrature (e.g., imaginary) component. In onearrangement, generating an indication of tissue proximity may be basedon one of the components of the complex impedance value. In one specificarrangement, the real component of an impedance value may be compared toa real component of a baseline impedance value to provide an indicationof tissue contact. In this arrangement, the quadrature components of theimpedance values may be compared to determine interference resultingfrom, for example, contact and/or proximity to structures other thantissue. By way of example, the quadrature components of the impedancevalues may be used to identify when an electrode is sheathed orunsheathed and/or when an electrode contacts another electrode.

In either of the noted systems or methods, subsequent impedance valuesmay be utilized to generate an indication of a change in patient tissue.By way of example, a change between a subsequent impedance value and abaseline impedance value may provide an indication of lesion formationin patient tissue during an ablation procedure.

In a further arrangement, a system and method are provided fordetermining spatially dependent baseline impedance values. In thisarrangement, baseline impedance values are determined for specificlocations within, for example, a three-dimensional space such as aninternal patient cavity. Subsequent changes of impedance values areassessed on a location-by-location basis rather than being assessed onan electrode-by-electrode basis. The method includes identifying alocation of each electrode of a medical device in the three-dimensionalspace. That is, each electrode may be identified within a sub-region ofthe three-dimensional space. The sub-regions may be defined as a grid orother sub-division of the three-dimensional space. Drive signals areapplied to each of the electrodes of the medical device and responses ofthe electrodes to the drive signals are measured. Impedance values aregenerated for each electrode. Based on location of the electrode in thethree-dimensional space and the impedance value for that electrode,sub-regions of the three-dimensional space are assigned baselineimpedance values. That is, a sub-region containing an electrode isassigned the impedance value for that electrode as a baseline impedancevalue. Subsequent impedance values for an electrode located in thesub-region are compared to the baseline impedance value for thatsub-region. If the subsequent impedance value for a sub-region isgreater than the baseline impedance value for the sub-region, anindication of tissue proximity between the electrode and tissue may begenerated. If the subsequent impedance value for the sub-region is lessthan the baseline impedance value for the sub-region, the baselineimpedance value may be reset to the subsequent impedance value. Thesystem and method may further include moving electrodes of the medicaldevice throughout the three-dimensional space to assign baselineimpedance values to most or all sub-regions of the three-dimensionalspace. The method may also be used to generate indications of lesionformation in each sub-region during, for example, an ablation procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of an exemplary embodiment of an elongatemedical device having a distal lariat portion.

FIG. 2 is an end view of the distal end portion of the elongate medicaldevice of FIG. 2 , illustrating multiple electrodes that may be used onthe device.

FIG. 3 is a diagrammatic view of an exemplary embodiment of a system fordetermining an impedance at two electrodes on an elongate medicaldevice.

FIG. 4 is an end view of a distal end portion of an alternate embodimentof an elongate medical device illustrating a high electrode count.

FIG. 5 is a chart illustrating a sum current as a function of channelcount.

FIG. 6 is a chart illustrating a maximum number of channels vs. current.

FIG. 7 is a diagrammatic depiction of an exemplary system that mayinclude functionality for determining impedances for a plurality ofpairs of electrodes on an elongate medical device.

FIG. 8 is a diagrammatic depiction of one embodiment of a currentsource.

FIG. 9 is a diagrammatic depiction of one embodiment of a measurementcircuit and demodulation circuit.

FIG. 10 is a diagrammatic depiction of an embodiment of a current sourceconfigured to provide a plurality of drive signals to electrodes of amedical device.

FIG. 11 is a diagrammatic depiction of an embodiment of a measurementcircuit and demodulation circuit configured to measure and demodulateresponses from a plurality of electrodes of a medical device.

FIG. 12 is a chart illustrating a sum current of a plurality of harmonicfrequencies.

FIG. 13 is a chart illustrating a sum current when a plurality ofharmonic frequencies have random phase offsets.

FIG. 14 is a flow chart illustrating a process for use with thedisclosed systems.

FIG. 15 is a flow chart illustrating another process for use with thedisclosed systems.

FIG. 16A illustrates a medical device contacting patient tissue.

FIG. 16B illustrates a simplified electrical circuit of FIG. 16A.

FIGS. 17A-17C illustrate three levels of contact between a medicaldevice and patient tissue.

FIG. 18 illustrates a medical device disposed within a heart chamber.

FIG. 19 is a flow chart illustrating a first process for use todetermine baseline impedance values.

FIG. 20 is a flow chart illustrating a second process for use todetermine baseline impedance values.

FIG. 21 illustrates dividing an internal patient cavity into sub-regionsfor baseline impedance determination.

FIG. 22 is a flow chart illustrating a third process for use todetermine baseline impedance values.

FIGS. 23A and 23B illustrate contact maps showing a geometry of aninternal patient cavity.

FIG. 24 illustrates a quadrature impedance response of an electrode fordifferent contacts.

FIG. 25 is first graph of impedance responses of electrodes over time.

FIG. 26 is a second graph of impedance responses of electrodes overtime.

DETAILED DESCRIPTION

Referring now to the figures, in which like numerals indicate the sameor similar elements in the various views, FIG. 1 is an isometric view ofan exemplary embodiment of an elongate medical device 24. The elongatemedical device 24 may comprise, for example, a diagnostic and/or therapydelivery catheter, an introducer or sheath, or other like devices. Forpurposes of illustration and clarity, the description below will be withrespect to an embodiment where the elongate medical device 24 comprisesa catheter (i.e., catheter 24). It will be appreciated, however, thatembodiments wherein the elongate medical device 24 comprises an elongatemedical device other than a catheter remain within the spirit and scopeof the present disclosure.

Referring to FIG. 1 , the catheter 24 may comprise a shaft 28 having adistal end portion 26 and a proximal end portion 30. The catheter 24 maybe configured to be guided through and disposed in the body of apatient. Accordingly, the proximal end portion 30 of the shaft 28 may becoupled to a handle 32, which may include features to enable a physicianto guide the distal end portion to perform a diagnostic or therapeuticprocedure such as, for example only, an ablation or mapping procedure onthe heart of the patient. Accordingly, the handle 32 may include one ormore manual manipulation mechanisms 34 such as, for example, rotationalmechanisms and/or longitudinal mechanisms, coupled to pull wires fordeflecting the distal end portion of the shaft. Exemplary embodiments ofmanipulation mechanisms, pull wires, and related hardware are described,for example only, in U.S. patent application publication no.2012/0203169, hereby incorporated by reference in its entirety. Thehandle 32 may further include one or more electromechanical connectorsfor coupling to a mapping and navigation system, an ablation generator,and/or other external systems. The handle 32 may also include one ormore fluid connectors 36 for coupling to a source and/or destination offluids such as, for example only, a gravity feed or fixed orvariable-rate pump. Accordingly, the distal end portion 26 of the shaft28 may also include one or more fluid ports or manifolds fordistributing or collecting fluids such as, for example only, irrigationfluid during an ablation procedure. The fluid ports may be fluidlycoupled with one or more fluid lumens extending through the shaft 28 tothe handle 32. In some embodiments, the elongate medical device 24 maycomprise an introducer that includes at least one lumen configured toreceive another device such as a catheter or probe.

The distal end portion 26 of the shaft 28 of the exemplary catheter 24may have a lariat shape. See also FIG. 2 . In this embodiment, thelariat shape may be formed by, for example, a shape memory wire disposedwithin the shaft. A tip electrode 22 and a number of ring electrodes20A, 20B, 20C, 20D, 20E, 20F, 20G, 20H, 20I (which, may be referred toherein individually and generically as a ring electrode 20 or in themultiple as the ring electrodes 20) may be disposed on the distal endportion 26 of the shaft 28. For example, the tip electrode 22 and ringelectrodes 20 may be disposed on the lariat portion of the shaft 28. Inthe illustrated embodiment, the distal end portion 26 includes nine (9)ring electrodes 20 (i.e., a “decapolar” catheter having ten totalelectrodes, as illustrated in FIG. 2 ). In other embodiments, the distalend portion 26 includes nineteen (19) ring electrodes 20 (i.e., a“duo-decapolar” catheter having twenty total electrodes). The electrodes20, 22 on the catheter 24 illustrated in FIGS. 1 and 2 may be used forapplying ablation energy to tissue, acquiring electrophysiology datafrom tissue, determining the position and orientation (P&O) of theshaft, and/or other purposes. The electrodes 20, 22 may be coupled toelectrical wiring within the shaft 28, which may extend to the handle 32and to electromechanical connectors for coupling to external systems.The ring electrodes 20 may be placed in pairs, in one non-limitingembodiment, with two electrodes 20 in a pair disposed a first distanceaway from each other along the length of the shaft 28, and second pairof electrodes 20 separated by a second distance along the length of theshaft 28. For example, electrodes 20B and 20C (e.g., bi-pole pair ofelectrodes) may be considered a first pair, electrodes 20D and 20E maybe considered a second pair, and so on. These distances may be equal, orthe first distance may be different than the second distance. It will beappreciated that the catheter, 24 illustrated in FIGS. 1 and 2 isexemplary in nature only. The teachings of the present disclosure mayfind use with numerous other medical devices, such as circular mappingcatheters, other known mapping and diagnostic catheters, and other knownmedical devices.

An elongate medical device having multiple electrodes, such as thecatheter 24, may find use in a system for assessing a state of contactbetween the elongate medical device and the tissue of a patient. Asmentioned in the Background, in some known systems, an electricalcurrent may be driven between an electrode on an elongate medical devicedisposed within the body and a cutaneous electrode to assess suchcontact. The electric potential on the in-body electrode may be measuredwith reference to a third electrode (e.g., another cutaneous electrode),and an impedance may be calculated, where such an impedance may indicatea contact state. Such a uni-polar system and methodology may be improvedupon by a system for assessing a contact state according to anelectrical current driven between two electrodes on the same device(e.g., on the same elongate medical device within the patient's body).That is, impedance may be measured between a pair of electrodes (e.g.,bi-pole pair of electrodes), on the same device, eliminating artifactsthat may appear in a uni-polar arrangement. For instance, in a uni-polararrangement some of the current between the internal electrode and theexternal electrode must pass through the lungs of a patient, whichchanges impedance with each breath.

FIG. 3 is a diagrammatic view of a system 40 for assessing a contactstate according to an electrical current driven between two electrodes(e.g., a bi-pole electrode) on the same device. The system 40 mayinclude a medical device 42 comprising at least two electrodes A, Bhaving respective impedances Z_(A), Z_(B), a detection amplifier 44, anda signal generator 46. The detection amplifier may include, in onenon-limiting embodiment, two operational amplifiers (op amps) 52 _(A),52 _(B), a reference electrode R, and a measurement circuit or impedancesensor, which may be part of an electronic control unit (ECU) 50. In oneembodiment, the signal generator may be incorporated in or may beconsidered a part of the ECU.

The medical device 42 may be or may include an elongate medical devicesuch as the catheter 24 (see FIG. 1 ). The electrodes A, B may be anytwo electrodes on the device. For example, referring to FIG. 2 , theelectrodes A, B may be the tip electrode 22 and the first ring electrode22A. Alternatively, the electrodes A, B may be two ring electrodes 20Dand 20E, or 20F and 20G, etc.

The signal generator 46 may be configured to generate (e.g., among othersignals), a drive signal or excitation signal across the electrodes A, B(i.e., using one electrode as a source and the other as a sink). In oneembodiment, the drive signal may have a frequency within a range fromabout 1 kHz to over 500 kHz, more typically within a range of about 2kHz to 200 kHz, and even more typically about 20 kHz. In one embodiment,the drive signal may be a constant current signal, typically in therange of between 20-200 μA, and more typically about 100 μA.

The ECU 50 may include conventional filters (e.g., bandpass filters) toblock frequencies that are not of interest, but permit appropriatefrequencies, such as the drive frequency, to pass, as well asconventional signal processing software used to obtain the componentparts of the measured complex impedance. Accordingly, the ECU 50 mayinclude a memory storing such signal processing software and a processorconfigured to execute the signal processing software. The ECU 50 mayinclude any processing apparatus such as, as noted above, a memory and aprocessor. Additionally, or alternatively, the impedance sensor mayinclude an application-specific integrated circuit (ASIC), programmablelogic device (PLD), field-programmable gate array (FPGA), and/or otherprocessing device.

The detection amplifier 44 may have a positive polarity connector (e.g.,first channel) which may be electrically connected to a first electrodeA and a negative polarity connector (e.g., second channel) which may beelectrically connected to a second electrode B. The positive andnegative polarity connectors may be disposed relative to the othercomponents of the detection amplifier 44 so as to form the circuitdiagrammatically shown in FIG. 3 when connected with the electrodes A,B. It should be understood that the term connectors as used herein doesnot imply a particular type of physical interface mechanism but israther broadly contemplated to represent one or more electrical nodes.

The detection amplifier may drive a current between electrodes A, B onthe same device to assess a contact state between the electrodes A, Band tissue. Impedances may be calculated based on those driven currentsto determine a contact state. The system may be configured to determineimpedances respective of the first and second electrodes A, B todetermine a contact state.

Determination of impedances may begin with driving a sinusoidalelectrical signal (e.g., drive signal or excitation signal) betweenelectrodes A and B, with one of electrodes A and B selected as a source,and the other as a sink. The source and sink selection may be made bythe ECU 50, and the current driven by the signal generator 46. The drivesignal may have predetermined characteristics (e.g., frequency andamplitude). Electrical potentials are measured on electrodes A and Bwhile driving the current between electrodes A and B. The potentials maybe measured by a detection amplifier, in an embodiment. The detectionamplifier may present a very high impedance (for example, about 100 kΩor more, in an embodiment, and/or 50 times or more greater than thenominal impedance of one of the electrodes A, B, in an embodiment,and/or 100 times or more greater than the nominal impedance of one ofthe electrodes A, B, in an embodiment) relative to the path betweenelectrodes A and B, so the effect of measurements with the detectionamplifier on the potential on the electrodes A, B may be negligible.

Measurement may further include referencing the measured electricpotentials to a reference electrode, such as electrode R (shown in FIG.3 ). Reference electrode R may be a cutaneous electrode, such as a bodypatch electrode, in an embodiment. Alternatively, the referenceelectrode R may be another in-patient electrode. Such referencing may beperformed by inputting the potential on electrode A into a first inputof the first op amp 52 _(A), the potential on electrode B into a firstinput of the second op amp 52 _(B), and the potential on the referenceelectrode into respective second inputs on both the first op amp 52 _(A)and the second op amp 52 _(B). The output of the op amps 52 _(A), 52_(B) may be input to the ECU 50 for impedance determinations, contactassessment, and/or other calculations. In another embodiment, hardwareseparate from the ECU 50 may be provided to perform some or all of theimpedance and/or contact determinations.

For driving the current between electrodes A and B and determiningelectric potentials on electrodes A and B, known methods of driving acurrent at a particular carrier frequency and demodulating therespective potentials on electrodes A and B may be used. The detectionamplifier may amplify the signals manifest on each electrode A, B, andafter demodulation a voltage related to the impedance of each electrodeis available. In the case of electrode B, the recovered voltage will benegative (i.e., assuming electrode A is selected as the source and B asthe sink), so a conversion to a positive quantity may be applied by theECU 50 or other device. Since the current source-sink electrode pair maycomprise a closely spaced bi-pole, the potential at the referenceelectrode R with respect to the bi-pole will be similar, and thus thephysical location of R may vary with little effect on the voltagesbetween A and R and B and R.

For a given electrode geometry for which impedance is measured at asufficiently high frequency, the potential measured for a current drivenbetween electrodes A, B may be essentially resistive in a pure saline orblood medium, and may reflect the electrode's geometry and the solutionconductivity. For example, a spherical electrode in a homogenous mediumwill have an electric potential for a current driven through theelectrode according to equation (1) below:

$\begin{matrix}{V = \frac{\rho\; l}{4\;\pi\; r}} & (1)\end{matrix}$where V is the electric potential, I is the applied current, p is themedia resistivity, and r is the distance from the center of theelectrode at which the potential measurement is made. The measuredimpedance may be taken as the measured potential on the electrodedivided by the applied current, as set forth in equation (2) below:

$\begin{matrix}{Z = \frac{V}{I}} & (2)\end{matrix}$Calculation of impedance based on electrode geometry is well known.Along these lines, equations for ring electrodes and/or conversions froma spherical electrode to a ring electrode are known. Further, the effectof the influence of one electrode (e.g., A) on another electrode (e.g.,B) of and electrode pair can be calculated and accounted for. Exemplaryembodiments for calculating impedance based on electrode geometry andaccounting for effects of influence of an adjacent electrode aredescribed, for example only, in U.S. patent application publication no.2014/0364715, hereby incorporated by reference in its entirety.

For each potential measured as a current is driven between electrodes Aand B, geometry specific equations may be solved (e.g., by the ECU 12)to determine the voltages on each of electrodes A and B (relative toreference electrode R). Accordingly, such equations may be stored in thememory of the ECU 50 for execution by the processor of the ECU 50. Thosevoltages may then be applied to equation (1) or another geometricspecific equation to determine impedances respective of each ofelectrode A and B (again, by the ECU 50, for example). Based on thoseimpedances, a contact state between electrodes A and B and the tissue ofa patient may be assessed. Such measurements may be carried out numeroustimes. Furthermore, such measurements may be carried out for numeroussets of electrodes A and B. That is, impedance potentials may be carriedout repeatedly for numerous different pairs of electrodes to determine acontact state for each of those electrodes. For example, referring toFIGS. 1 and 2 , the measurements may first be carried out on electrodes22 and 20A, then on 20B and 20C, then on 20D and 20E, and so on. Statedotherwise, the impedance of pairs of electrodes may be determinedsequentially.

While contact assessments based on a current driven between electrodepairs on a catheter (or other medical device) provides increasedaccuracy in comparison to contact assessments based on a current drivenbetween an electrode on a catheter and an exterior/cutaneous electrode,aspects of the present disclosure are based, in part, on the realizationthat previous contact assessment systems have limitations. One specificlimitation is that medical standards establish current limits (auxiliarycurrent) for medical devices. For instance, such industry standardsallow for 10 micro-amps of current for an intra-cardiac electrode for ACcurrents below 1 kHz. At 10 kHz, the limit is 100 micro-amps withproportionally increasing limits with increasing frequency (i.e., at 20kHz the limit is 200 micro-amps). The auxiliary current limitation(e.g., threshold) works against a current trend in electrode catheters.Namely, the increasing number of electrodes carried on a catheter (orother medical device) to improve, for example, mapping accuracy and/orablation control. By way of example, one existing electrode catheter,the FIRMmap basket catheter by Topera/Abbott Laboratories, utilizes 64separate electrodes. Other proposed catheters contain 100 or even 200separate electrodes. FIG. 4 illustrates a distal end of an exemplarycatheter 16 having 128 electrodes. 20 ₁-20 ₁₂₈. In the illustratedembodiment, the distal end of the catheter 16 is formed as an expandablebasket having eight arms 18 a-h. The arms 18 a-h may be formed fromshape metal wires such that they expand to the illustrated shape whendisposed through the end of, for example, an introducer. Each armincludes 16 electrodes forming eight pairs of electrodes. In the case ofsuch a 128-electrode catheter, impedance of 64 electrode pairs may besequentially determined. Such sequential determination of such a largenumber of electrode pairs reduces the response time of the system.Another solution is to simultaneously drive current across each pair ofelectrodes. However, if the electrodes pair are driven with a current ata common or single frequency, cross talk between the electrode pairsmakes identifying the response of any given pair of electrodes difficultor impossible. Additionally, driving a current across a plurality ofelectrode pairs at a single frequency results in additive auxiliarycurrent (e.g., at a surface electrode or other internal electrode). Forinstance, for an auxiliary current limit of 100 micro-amps (e.g.,excitation frequency at 10 kHz), a catheter having 40 electrodes (i.e.,20 bi-poles or pairs of electrodes) would be limited to using a 5micro-amp current (i.e., 20 bi-poles*5 micro-amps=100 micro-amps). Thesum current is an additive total of all of the channel pairs. For acatheter having 100 pairs of electrodes (e.g., bi-poles), the drivecurrent would be limited to 1.0 micro-amps. As the number of electrodesincreases, the magnitude of the drive current must decrease to maintainthe sum current below threshold auxiliary current limits. As will beappreciated, lowering the magnitude of the drive current applied acrosseach bi-pole reduces the signal-to-noise ratio of its response.Accordingly, for medical devices with high numbers of electrodes, theresponse of the bi-pole pairs of electrodes may be overwhelmed by noise.

Aspects of the present disclosure are further based on the recognitionthat utilization of multiple drive signals having multiple differentfrequencies (e.g., unique frequencies) allows for increasing themagnitude of the drive current for each pair of electrodes (e.g.,bi-pole) or increasing the number of bi-poles without exceedingauxiliary current limits/thresholds. That is, it has been recognizedthat the sum current where multiple bi-poles are excited by multipledrive signals each having different/unique frequencies rises with thesquare root of the number of channels. In such a configuration, thetotal measured current or sum current is:

$\begin{matrix}{{Itotal} = {{{Ifrequency}*\sqrt{Nfrequencies}} = {{Ifrequency}*\sqrt{( \frac{Nchannels2}{\square} )}}}} & (3)\end{matrix}$

Where Ifrequency is the current per frequency (i.e., per bi-poleelectrode pair) and Nfrequencies is the total number of frequencies.Note the total number of channels is twice the number of frequencies(since one frequency services a bi-pole electrode pair). For example,for a medical device or catheter having 200 electrodes, 100 differentfrequencies would be used. Assuming these frequencies are above 10 kHz(e.g., spaced every 25 Hz over a 2500 Hz band 15-17.5 kHz), drivesignals having a 5 micro-amp current would result in a sum current of nomore than 50 micro-amps (i.e., 5 micro-amps*√100), well below the 100micro-amp limit for 10 kHz. Of note, the actual safe current limit isgreater than 100 micro-amps as each additional frequency is higher thanthe previous frequency and thus greater than 10 kHz. However, the 100micro-amp limit is utilized for simplicity.

The reduction of the sum current resulting from use of multiple uniquefrequencies compared to the single frequency example discussed above(i.e., 100 bi-poles; 1 micro-amp drive current; 100 micro-amp sumcurrent) occurs in conjunction with a five-fold increase in themagnitude of the drive current (i.e., 5 micro-amps vs. 1 micro-amp).This is illustrated in the chart of FIG. 5 . As the chart demonstrates,the application of a single frequency drive signal with a 5 micro-ampdrive current, 100 micro-amps is reached at 40 channels (e.g., 20bi-pole at 5 micro-amps each) while only 50 micro-amps is reached at 200channels when using unique frequencies. The use of unique frequenciesprovides a significant advantage in increasing the total number ofelectrode for a medical device.

Equation (3) may be rearranged to find the maximum number of channelsfor a given drive current:

$\begin{matrix}{{Nchannels} = {2( \frac{Itotal}{Ifrequency} )^{2}}} & (4)\end{matrix}$Thus, with 5 micro-amps per bi-pole at 10 kHz and higher (using a flat100 micro-amp auxiliary limit/threshold for simplicity) the maximumnumber of channels is:

$\begin{matrix}{{2( \frac{100\mspace{14mu}{uA}}{5\mspace{14mu}{uA}} )^{2}} = {{2( 20^{2} )} = {800\mspace{14mu}{channels}}}} & (5)\end{matrix}$Conversely, doubling the current per bi-pole electrode pairs reduces themaximum number of channels as a function of its square. That is, whenusing a 10 micro-amp drive signal per bi-pole, 200 channels would beallowed. When using a 20 micro-amp drive signal per bi-pole, 50 channelswould be allowed. Stated otherwise, lowering current increases allowablechannel count by a square factor while increasing current decreasesallowable channel count by a square factor. FIG. 6 provides a chart thatillustrates how a 100 uA limit can be achieved with different currentlevels per bi-pole and associated channel count. As shown, below about15 micro-amps the number of allowable channels increases dramatically.This demonstrates the advantage of reducing the current per bi-pole. Offurther note, this also allows determining a maximum drive current basedon a number of bi-poles contained on or in a medical device. That is,the drive current may be maximized for a given number of bi-poles whileremaining within safe sum current limits to enhance signal-to-noiseratios of response signals.

Higher unique frequencies also assist in increasing the maximum numberof channels possible while maintaining safe sum current limits. For a200 micro-amp auxiliary current limit (e.g., for frequencies 20 kHz andup), the theoretical count increases to:

$\begin{matrix}{{2\;( \frac{200\mspace{14mu}{uA}}{5\mspace{14mu}{uA}} )^{2}} = {{2( 40^{2} )} = {3200\mspace{14mu}{channels}}}} & (6)\end{matrix}$Such a large number of channels may not be practical for many reasonsbut demonstrates the benefit of higher frequencies along with low drivecurrent per bi-pole pair. In any arrangement, use of unique frequenciesfor the drive signals of a plurality of bi-pole electrodes significantlyincreases the number of bi-poles that may be interrogated to determineimpedance. Alternatively, use of unique frequencies allows forincreasing the magnitude of a drive current applied to the bi-poleswhile maintaining auxiliary current limits for a patient below apredetermined threshold.

While utilizing unique frequencies for each drive signal providessignificant benefits for determining impedances of high-count electrodemedical devices, the measured response signal to the drive signals mustbe identified for each bi-pole. The disclosed method and system utilizedigital signal processing to synchronously demodulate the responsesignal (e.g., voltage signal) at each electrode. Another importantaspect of the present disclosure is that driving each electrodepair/bi-pole at a unique frequency not only allows for significantlyincreasing a number of electrodes that may be interrogated and/orincreasing drive current magnitudes but also minimizes crosstalk betweenchannels.

The following discussion is directed to an exemplary embodiment of amedical device having 200 electrodes (100 bi-poles) using 100 spaceddrive frequencies. By spacing these drive frequencies at exactly 25hertz apart, the bandwidth requirement is 25×100=2500 hertz. Otherfrequency offsets are possible. In this example drive frequencies from15025 Hz through 17500 are utilized. Keeping the frequencies tightlypacked simplifies bandwidth requirements of the digitizing amplifiercircuit. Further, each electrode pair/bi-pole is driven with a currentin the 1 to 10 micro-amp range. It will be appreciated that differentfrequency ranges and drive currant ranges may be utilized.

Synchronous demodulation allows the unique frequencies to be detectedindependent of each other while minimizing crosstalk. To achieve this,the drive frequencies are made orthogonal to each other by setting thedrive frequencies at harmonics of a base frequency (e.g., 25 Hz in thepresent example) and measuring a response over a period with an integernumber of cycles. By selecting an update/sampling rate of 25 per second(e.g., 40 millisecond period), frequencies on 25 hertz boundaries willhave integer number of cycles in each sampling period. That is,frequencies on 25 hertz boundaries such as 16025, 16050, 16075 hertzetc. will be orthogonal to each other. The sampling rate of 25 persecond was selected as a compromise between tight frequency packing andfast response time. For cardiac application, it is noted that a heartbeats in the range of 1 to 4 beats per second and 25 samples per secondis capable of tracking changes due to cardiac motion. It is possible tospace frequencies closer together, but the ability to track impedancechanges through the cardiac cycle diminishes. Reducing the spacing by afactor of 2 to 12.5 Hz would also reduce the reporting/sampling rate to12.5 per second and, while possible, is less than ideal for tracking theimpedance changes in a rapidly beating heart. Likewise, it is possibleto increase spacing and, in turn, achieve more samples per second,though bandwidth requirements increase.

Synchronous demodulation consists of multiplying the measured anddigitized response signal (which is a composite of multiple frequencies)by a replica of each drive signal of exactly the same frequency and aknown phase offset. The resultant signal is then low-pass filtered anddecimated to (in this example) 25 samples per second. The sampling rateof the analog-to digital converter (ADC) is not critical and in factneed not meet the traditional Nyquist sampling rate. However, theamplifying circuit must have adequate bandwidth to pass the signal tothe ADC. By calibrating the system and compensating for expected phasedelay between drive signal and received signal, quadrature demodulationmay occur. Thus, an in-phase component for resistive impedance and aquadrature component for reactive impedance may be found. This iscommonly known as complex impedance. Synchronous demodulation alsoallows for signal extraction with very low current levels. Successfuldetection of impedance below 1 micro-amp has been demonstrated, thoughhigher current levels provide better signal-to-noise ratio.

FIG. 7 is a diagrammatic depiction of an embodiment of an exemplarymapping and navigation system 70 that be utilized with an elongatedmedical device 16 to, for example, determine impedance, determinecontact sensing, determining the location (i.e., position andorientation) of an elongate medical device (e.g., catheter) within thebody of a patient, mapping the anatomy of the patient, etc. The system70 may include various visualization, mapping and navigation componentsas known in the art, including, for example, an EnSitePrecision™ systemcommercially available from St. Jude Medical, Inc., or as seengenerally, for example, by reference to U.S. Pat. No. 7,263,397, or U.S.patent application publication no. 2007/0060833, both of which arehereby incorporated by reference in their entireties as though fully setforth herein.

The system 70 may include an electronic control unit (ECU) 72, ananalog-to-digital converter (A-to-D) 74, a filter 76 (e.g., bandpassfilter), a digital to analog converter 84, a filter 86 (e.g., bandpassfilter), a switch 78, a signal source or signal generator 80, ademodulator circuit 130, a graphical user interface 68 and, in variousembodiments, a plurality of body surface patch electrodes 82. Additionalcircuitry may be included as more fully discussed below. The system 70may be electronically and/or mechanically coupled with an elongatemedical device such as the 128-electrode catheter 16 of FIG. 4 . Thesystem 70 may be configured for a number of functions for guiding theelongate medical device 16 to a target site within the body of a patient98, such as the heart 92, and for assessing contact between the elongatemedical device 84 and the tissue of the patient 98. The system 70 mayfurther include a conventional set of EGC leads 90 for the capture andmeasure patient ECG data. The elongate medical device may be one of thecatheters 24 or 16 described herein (see FIGS. 1 and 4 ), or some otherelongate medical device. The elongate medical device may have aplurality of pairs of electrodes.

The signal generator 80 outputs multiple excitation or drive signals forassessing an impedance of one or more electrodes. More specifically, thesignal generator 80 may generate a plurality of excitation or drivesignals having unique frequencies within a range from about 1 kHz toover 500 kHz, more typically within a range of about 2 kHz to 200 kHz,and even more typically between about 10 kHz and about 20 kHz, in oneembodiment. The drive signals may each have a constant current,typically in the range of between 1-200 μA, and more typically about 5μA, in one embodiment. The signal generator 80 may also generate signalsinvolved in, for example, determining a location of the electrodes 92within the body of the patient.

The ECU 72 may include a memory 94 and a processor 96. The memory 94 maybe configured to store data respective of the elongate medical device84, the patient 98, and/or other data (e.g., calibration data). Suchdata may be known before a medical procedure (medical device specificdata, number of catheter electrodes, etc.), or may be determined andstored during a procedure. The memory 94 may also be configured to storeinstructions that, when executed by the processor 96 and/or a contactassessment module 116, cause the ECU 72 to perform one or more methods,steps, functions, or algorithms described herein. For example, butwithout limitation, the memory 94 may include data and instructions fordetermining impedances respective of one or more electrodes 92 on theelongate medical device 84. The ECU may be connected to a graphical userinterface 68, which may display an output of sensed tissue (e.g.,heart), the elongated medical device (not shown) and/or assessed values(e.g., impedances) for electrodes of the elongated medical device.

FIG. 8 illustrates one embodiment of a signal source 80 (e.g., currentsource) that provides an excitation signal for one pair of electrodes.In the present embodiment, the signal source 80 includes a fieldprogrammable gate array (FPGA) 88. However, it will be appreciated thatother circuitry, including without limitation, application specificintegrated chips, Altera Cyclone series or Xilinx Spartan series may beutilized. In the present embodiment, the FPGA 88 includes a numericallycontrolled oscillator (NCO) 102. The NCO 102 is a digital signalgenerator which creates a synchronous (i.e. clocked), discrete-time,discrete-valued representation of a waveform, usually sinusoidal. TheNCO 102 is programmable to provide a waveform having a desiredfrequency, amplitude and/or phase.

In the present embodiment, the NCO 102 creates a sinusoidal waveform ofa desired frequency based on an input (e.g., single fixed-frequencyreference) provided from a microprocessor and/or control logic 104. Inthe present embodiment a microprocessor/control logic 104 isincorporated in the FPGA provides the inputs to the NCO 102. However, itwill be appreciated that the NCO inputs may be provided by, for example,the processor 96 of the ECU 72. In any arrangement, the NCO 102generates a digital waveform output having a desired frequency (e.g.,unique frequency). The output of the NCO is received by a digital toanalog converter (DAC) 106, which converts the received digital signalto a corresponding analog signal. A bandpass filter 108 is utilized tosmooth the converted analog signal. A differential driver (e.g., op amp)110 receives the smoothed analog signal from the bandpass filter 108 andsends the same signal as a differential pair of signals, each in its ownconductor to an isolation transformer 112. Provided that impedances inthe differential signaling circuit (e.g., differential driver andisolation transformer) are equal, external electromagnetic interferencetends to affect both conductors identically. As the receiving circuit(isolation transformer) only detects the difference between theconductors, the technique resists electromagnetic noise compared to aone conductor arrangement. The isolation transformer 112 transfers ACcurrent of the signals originating from the source 80 to the electrodesA and B of the medical device while isolating the medical device fromthe source. The isolation transformer 112 blocks transmission of DCcomponents in the signals from passing to the electrodes while allowingAC components in signals to pass. The dual output from the isolationtransformer 112 is received by AC coupler 114 (e.g., capacitor) thatfurther limit low frequency current from passing to the electrodes. TheAC coupler outputs the signals to the electrodes A and B of theelectrode pair (e.g., bi-pole). The AC coupler 114 has an impedance thatis orders of magnitude greater than the impedance across the electrodesA and B.

FIG. 9 illustrates one embodiment of a signal measuring circuit (e.g.,signal sampler) and a synchronous demodulation circuit. Initially, aresponse signal from one of the electrodes A or B is received at afilter 120 (e.g., buffer amplifier) that transfers a current from theelectrode, which has a low output impedance level, to an analog todigital converter (ADC) 122, which typically has a high input impedancelevel. The buffer amplifier prevents the second ADC from loading thecurrent of electrode circuit and interfering with its desired operation.The ADC 122 samples the received analog signal at a known sampling rate(e.g., 64 k/s) and converts the analog response signal to a digitalresponse signal. In the present embodiment, an output of the ADC passesthrough a digital isolator 124, which transfers the digital responsesignal to the control system (e.g., ECU) while isolating the controlsystem from the medical device.

The digital response signal passes to a synchronous demodulator circuit130 which, in the present embodiment, is defined in the same FPGAutilized for the signal source 80. As noted, synchronous demodulationconsists of multiplying a digitized response signal by a replica of adrive signal of exactly the same frequency and a known phase offset.That is, a demodulation signal having the same frequency as the drivesignal and a known phase offset from the drive signal is generated andmultiplied with the digitized response signal. Generating thedemodulation signal(s) using the same FPGA 88 that generates the drivesignal(s) simplifies the demodulation process. However, it will beappreciated that this is not a requirement and that the synchronousdemodulator circuit and the signal source may be separate and/or formedof different software and/or hardware components. In any arrangement,the synchronous demodulation circuit must be able to replicate the drivesignal for a given frequency.

In the illustrated embodiment, the digital response signal is split asit is received by the synchronous demodulator circuit 130. A numericallycontrolled oscillator (NCO) 132 generates sine and cosinerepresentations of the corresponding drive signals. Each signal isadjusted with a phase delay such that the cosine signal aligns with thein-phase (e.g. resistive) component based on an input provided from themicroprocessor and/or control logic 104. The split digital responsesignals are multiplied point-by-point by the sine and cosine signals insine and cosine multipliers 134, 136, respectively. This yields in-phaseand quadrature channels. The channels are filtered and decimated by lowpass decimating filters 138, 140, which in the present embodiment areformed of cascaded integrator-comb (CIC) filters. Following the exampleabove, where the drive signal is a harmonic of a 25 Hz base frequency,the channels/signals are decimated to 25 samples per second such thateach decimated signal has an integer number of cycles. The decimatedsignals then pass through a gain and offset calibration 142, 144 tocompensate for expected hardware variations and to scale the result toresistive and reactive impedance in units of ohms. This information maythen be transmitted, for example, via an output port 146 to, forexample, the ECU. The above noted measuring and demodulation process maybe performed for the responses of both electrodes A and B.

In order to accommodate a plurality of electrodes, the systems andprocesses of FIGS. 8 and 9 may be scaled. FIG. 10 illustrates anembodiment of a signal source 80 (e.g., current source) scaled toprovide a plurality of unique frequency excitation/drive signals for aplurality of electrode pairs/bi-poles. In the illustrated embodiment,the current source 80 provides 64 unique frequencies to 128 totalelectrodes (i.e., 64 electrode pairs/bi-poles). It will be appreciatedthat this embodiment is provided by way of example and not by way oflimitation. Along these lines, unique frequency drive signals may beprovided for more or fewer frequencies and/or electrodes. Similar to thesignal source described above in relation to FIG. 8 , the signal source80 is defined within a field programmable gate array (FPGA) 88. The FPGA88 further includes a plurality of numerically controlled oscillators(NCOs) 102 a-h (hereafter NCO 102 unless specifically referenced). Asabove, the NCOs 102 receive a reference signal input from amicro-compressor and/or control logic 104. In the illustratedembodiment, each NCO 102 has eight channels. That is, each NCO 102 isprogrammable to provide eight unique frequencies. In this regard, theeight NCOs 102 a-h are operative to provide 64 unique frequencies.Continuing with the previous example, each NCO provides eight uniquefrequencies spaced on 25 Hz intervals. Collectively, the NCOs 102 a-hprovide 64 individual frequencies between approximately 16 kHz and 18kHz. The output of each NCO 102 is received by a digital to analogconverter (DAC) 106 a-h. Each DAC has eight independent channels eachconfigured to generate an analog representation of a received drivesignal frequency for receipt by the electrodes of an attached medicaldevice. Similar to the source described in relation to FIG. 8 , theoutput of the DAC's may be received by bandpass filters, differentialdrivers and/or transformers prior to being applied to individualelectrodes of the medical device.

FIG. 11 illustrates one embodiment of a multi-channel signal measuringcircuit and multi-channel synchronous demodulation circuit. The overalloperation of the embodiment of FIG. 11 is similar to the operation ofthe embodiment of FIG. 9 . Initially, response signals from theelectrodes are received at a filter (e.g., buffer amplifier) thattransfers a current from the electrodes to analog to digital converters(ADCs) 122 a-h (hereafter 122 unless specifically referenced). As withthe DAC's of the signal source, the measurement circuit utilizes NCOs togenerate sine and cosine signals for synchronous demodulationcorresponding to each electrode's driven frequency.

A synchronous demodulator circuit 130 receives the digital responsesignals from the ADCs 122. In the present embodiment, the synchronousdemodulator circuit 130 is defined in the same FPGA utilized for thesignal source 80. More specifically, the digital signals are received bya 128-channel sequencer 194 which samples all the signals at one pointtime and provides the sampled signals to a pipelined multiplier 198. Thepipelined multiplier is in communication with a plurality of NCOs 132a-h, which again generate appropriately phase delayed sine and cosinerepresentations of each unique frequency drive signal based on inputsfrom the microprocessor and/or control logic 104. The pipelinedmultiplier 198 operates in a manner that is substantially identical tothe multipliers described above in relation to FIG. 9 with the exceptionthat pipelining allows the calculations of all channels utilizing asingle instantiation of demodulator circuit 130. The pipelinedmultiplier 198 multiplies each response by respective sine and cosinedemodulation signals. The output of the pipeline multiplier 198 isprovided to a pipelined low pass decimating filter 202, which samplesthe outputs over an integral number of cycles, as described above. Thedecimated signals then pass through pipelined gain and offsetcalibration 204 to convert to units of ohms impedance. Thus, a realcomponent of resistive impedance and an imaginary component of reactiveimpedance may be found for each of the 128 electrodes. This informationmay then be transmitted to the ECU.

The systems and processes of FIGS. 8-11 allow for synchronousdemodulation of a large number of electrodes when the drive signals areorthogonal (i.e., unique drive frequencies at harmonics of a basefrequency and responses are measured over a period with an integernumber of cycles). Further, the use of multiple unique drive frequenciesallows increasing the drive current as the RMS (root mean square) valueof the resultant signal increases with the square root of the number ofchannels. What is not obvious is the signals may add in a manner thatcreates a large peak value periodically when all the frequencies comeinto phase. One aspect of the synchronous demodulation scheme is thefrequencies are not random but are chosen to be separated by a fixedamount. As such, if all the frequencies start at time zero and are on 25Hz boundaries (or other equal boundaries), every 40 milliseconds (25times per second) all signals will be nearly in phase and a largeinstantaneous peak value results. This is shown graphically in FIG. 12which shows a trace of a sum current 300 where 200 frequencies startingat 16025 Hz and every 25 Hz thereafter to 19500 Hz are added together.Mathematically, the RMS current in this case of 200 frequencies at 5micro-amps is (5×√200) or about 70 micro-amps. However, there is a peak302 in the sum current 300 of about 1 milliamp every 40 milliseconds.Such a peak current would be expected to exceed an auxiliary currentlimit/thresholds. This can be countered by adding a random (non-uniform)phase offset to each channel. This minimizes the peaking and spreads thecurrent over time. The sum current 400 with each channel assigned arandom phase, is illustrated in FIG. 13 .

As shown in FIG. 13 , when random phase offsets are applied to eachdrive signal, the sum current 400 is much more uniform and lower in peakcurrent. The addition of phase offset to each frequency of the drivesignals does not hinder the synchronous demodulation as the drivesignals remain orthogonal at 25 Hz apart. The phase offset iscompensated during demodulation by simply phase delaying each input(e.g., reference frequency) in the FPGA as necessary at calibrationtime. This is facilitated by the use of separate NCOs for the signalsource and demodulation circuit, which have both frequency and phaseinputs. The source NCOs are assigned a one-time random phase offset thatmay be stored by the ECU and/or the FPGA. The demodulation NCOs areassigned a respective phase offset during a one-time calibration thatcompensates for the source NCO phase offset plus any phase delay betweensource NCO 102 and analog-to-digital convertor 122. Said calibrationdata is likewise stored by the ECU and/or FPGA.

FIG. 14 illustrates a process 320 that may be performed by the systemsdescribed above. Initially, the process includes generating 322 aplurality of drive signals each having a unique frequency that is aharmonic of a common base frequency. The generation of such a pluralityof drive signals may further entail assigning each drive signal a randomphase offset. Once the drive signals are generated, the drive signalsare simultaneously applied 324 across individual pairs of electrodes ofa medical device. The application of the drive signals may furtherinclude digital to analog conversion of the drive signals prior to theirapplication to the electrodes. One or more composite response(s) of theelectrodes to the drive signals is measured 326. The measurement mayfurther entail converting analog responses of the electrodes to digitalsignals. The digital signals are then synchronously demodulated 328. Thesynchronous demodulation entails generating demodulation signals foreach unique frequency. Each demodulation signal will have the samefrequency and a known phase offset for a corresponding drive signal. Ifeach drive signal has a random phase offset, the correspondingdemodulation signals will have the same additional random phase offset.The synchronous demodulation of the drive signals may also includesampling the signals over a time period that includes an integer numberof cycles for the drive signals. The synchronous demodulation outputs330 a complex impedance value for each electrode. That is, a realimpedance value and a reactive impedance value may be output for eachelectrode. For instance, these outputs may be output to the graphicaluser interface 68 (see FIG. 7 ). Along these lines, the assessed valuefor each electrode may be displayed on the graphical user interface 68along with a graphical depiction of the catheter to provide a user withfeedback on the contact status of each electrode. That is, the impedancevalues may be utilized for, among other things, to assess electrodecontact with tissue.

FIG. 15 illustrates a further process 340 that may be performed by thesystems described above. The process allows for dynamically adjustingcurrent levels of drive signals applied to a plurality of electrodes ofa medical device. Initially, the process includes determining 342 anumber of electrodes of an attached medical device. Such determinationmay be performed by a control unit (e.g., ECU) interrogating theattached medical device. Alternatively, a system user may input thisinformation. Based on the number of electrodes and a frequency band fora plurality drive signals that will be applied to the electrodes, anauxiliary current limit or threshold is determined 344. The auxiliarycurrent limit may be determined from stored data (e.g., calibrationdata). Based on the auxiliary current limit and the number ofelectrodes, a current level may be identified 346 for drive signals thatwill be applied to electrodes. For instance, the current level of thedrive signals may be maximized to enhance the signal-to-noise responseof the signals when applied to the electrodes while maintaining a sumcurrent of the drive signals below the auxiliary current limit. Once thecurrent level for the drive signals is identified, unique frequencydrive signals are applied 348 to the electrodes where each drive signalhas identified current level.

The systems described above provides further benefits for use withmedical devices. For instance, utilization of the DACs to generate thedrive signals provides a means for deactivating a channel. In thisregard, simply setting a DAC to zero or a static value effectively turnsoff a channel. Along these lines, channels may be purposefullydeactivated to permit increased current levels for drive signals ifneeded. Another benefit is provided by the bandpass filters. As thebandpass filters only permit passage of a narrow frequency range, anysoftware or hardware errors that result in outputting a drive signal oftoo low a frequency is not passed. The bandpass filters thus provide afail-safe limit to the drive signals.

In addition to impedance calculations and contact state determinations,the system 70 may be configured to determine the position andorientation (P&O) of an elongate medical device 16 (e.g., of a distalend portion of a catheter) within the body of the patient 98.Accordingly, the ECU 72 may be configured to control generation of oneor more electrical fields and determine the position of one or moreelectrodes 92 within those fields. The ECU 72 may thus be configured tocontrol signal generator 80 in accordance with predetermined strategiesto selectively energize various pairs (dipoles) of body surface patchelectrodes 82 and catheter electrodes.

Referring again to FIG. 7 , a mapping and navigation functionality ofthe system 70 will be briefly described. The body surface patchelectrodes 82 may be used to generate axes-specific electric fieldswithin the body of the patient 98, and more specifically within theheart 92. Three sets of patch electrodes may be provided: (1) electrodes82 _(X1), 82 _(X2), (X-axis); (2) electrodes 82 _(Y1), 82 _(Y2),(Y-axis); and (3) electrodes 82 _(Z1), 82 _(Z2), (Z-axis). Additionally,a body surface electrode (“belly patch”) 82 _(B), may be provided as anelectrical reference. The body patch electrodes 82 _(X1), 82 _(X2), 82_(Y1), 82 _(Y2), 82 _(Z1), 82 _(Z2), 82 _(B) may be referred to hereingenerically as a body patch electrode 82 or as the body patch electrodes82. Other surface electrode configurations and combinations are suitablefor use with the present disclosure, including fewer body patchelectrodes 82, more body patch electrodes 82, or different physicalarrangements, e.g. a linear arrangement instead of an orthogonalarrangement.

Each patch electrode 82 may be independently coupled to the switch 78,and pairs of patch electrodes 82 may be selected by software running onthe ECU 72 to couple the patch electrodes 82 to the signal generator 80.A pair of electrodes, for example the Z-axis electrodes 82 _(Z1), 82_(Z2), may be excited by the signal generator 80 to generate anelectrical field in the body of the patient 86 and, more particularly,within the heart 88. In an embodiment, this electrode excitation processoccurs rapidly and sequentially as different sets of patch electrodes 82are selected and one or more of the unexcited surface electrodes 82 areused to measure voltages. During the delivery of the excitation signal(e.g., current pulse), the remaining (unexcited) patch electrodes 82 maybe referenced to the belly patch 82 _(B) and the voltages impressed onthese remaining electrodes 82 may be measured. In this fashion, thepatch electrodes 82 may be divided into driven and non-driven electrodesets. A low pass filter may process the voltage measurements. Thefiltered voltage measurements may be transformed to digital data by theanalog to digital converter and transmitted to the ECU 72 for storage(e.g. in the memory 94) under the direction of software. This collectionof voltage measurements may be referred to herein as the “patch data.”The software may store and have access to each individual voltagemeasurement made at each surface electrode 82 during each excitation ofeach pair of surface electrodes 82.

Generally, in an embodiment, three nominally orthogonal electric fieldsmay be generated by the series of driven and sensed electric dipoles inorder to determine the location of the elongate medical device 16 (i.e.,of one or more electrodes). Alternately, these orthogonal fields can bedecomposed and any pair of surface electrodes (e.g., non-orthogonal) maybe driven as dipoles to provide effective electrode triangulation.

The patch data may be used, along with measurements made at one or moreelectrodes catheter electrode and measurements made at other electrodesand devices, to determine a relative location of the one or morecatheter electrodes. In some embodiments, electric potentials acrosseach of the six orthogonal patch electrodes 82 may be acquired for allsamples except when a particular surface electrode pair is driven. In anembodiment, sampling electric potentials may occur at all patchelectrodes 82, even those being driven.

As a part of determining locations of various electrodes, the ECU 72 maybe configured to perform one or more compensation and adjustmentfunctions, such as motion compensation. Motion compensation may include,for example, compensation for respiration-induced patient body movement,as described in U.S. patent application publication no. 2012/0172702,which is hereby incorporated by reference in its entirety.

Data sets from each of the patch electrodes 82 and the catheterelectrodes are all used to determine the location of the catheterelectrodes within the patient 98. After the voltage measurements aremade for a particular set of driven patch electrodes 82, a differentpair of patch electrodes 82 may be excited by the signal generator 80and the voltage measurement process of the remaining patch electrodes 82and catheter electrodes takes place. The sequence may occur rapidly,e.g., on the order of 100 times per second in an embodiment. The voltageon the catheter electrodes within the patient 98 may bear a linearrelationship with the position of the electrodes between the patchelectrodes 82 that establish the electrical fields, as more fullydescribed in U.S. Pat. No. 7,263,397, which is hereby incorporated byreference in its entirety.

In summary, FIG. 7 shows an exemplary system 70 that employs seven bodypatch electrodes 82, which may be used for injecting current and sensingresultant voltages. Current may be driven between two patches 82 at anytime. Positioning measurements may be performed between a non-drivenpatch 82 and, for example, belly patch 82 _(B) as a ground reference.The position of an electrode 92 may be determined by driving currentbetween different sets of patches and measuring one or more impedances.Some impedances that may be measured may be according to currents drivenbetween pairs or sets of two catheter electrodes on the elongate medicaldevice 16. In one embodiment, time division multiplexing may be used todrive and measure all quantities of interest. Position determiningprocedures are described in more detail in, for example, U.S. Pat. No.7,263,397 and publication no. 2007/0060833 referred to above.

As previously noted, impedance values may be utilized to assess contactbetween an electrode and patient tissue. Along these lines, measuringthe impedance of an electrode has been demonstrated to provide areliable method of detecting when that electrode comes in contact with,for example, intercardiac tissue. Specifically, due to the reducedconductivity of intercardiac tissue compared to blood, the impedance ofan electrode is significantly higher once it comes in contact with thetissue. Accordingly, the electrode impedances determined above may beutilized to provide an indication of tissue contact.

During a procedure, the impedance at an electrode-tissue interface,which is indicative of proximity or contact, may be measured before,during and/or after tissue contact using the measurement circuitdiscussed above and/or the contact assessment module 112 (see FIG. 7 ).The measured impedance, its resistive, reactance and/or phase anglecomponents or combination of these components may be used to determine aproximity or contact condition of one or all electrodes. The proximityor contact condition may then be conveyed to the user in real-time forachieving, for example, a desired level of contact and/or an indicationof lesion formation. Assessing a proximity or contact condition betweenan electrode of a medical device and target tissue based on impedancemeasurements at the electrode-tissue interface may be better understoodwith reference to FIGS. 16A and 16B, which show a model of an exemplaryelongated medical device 16 contacting tissue. As shown, in theillustrated embodiment, the elongated medical device 16 is a basket typeelectrode catheter having at least a first pair of electrodes 20 ₁ and20 ₂ in contact with target tissue 92 (e.g., cardiac tissue). Theelectrodes 20 ₁ and 20 ₂ are electrically connected to a signal source(e.g., signal source 80; see FIG. 7 ). Upon application of a drivesignal between the electrodes 20 ₁ and 20 ₂, a circuit may be completedbetween the electrodes such that current flows through blood and/orpatient tissue 92 (e.g., myocardium) as illustrated in FIG. 16A byarrows 150 and 152. The passage of at least a portion of the currentthrough the patient tissue at the electrode-tissue interface affects theinductive, capacitive, and resistive effects of the electrode responseto the drive signal(s). That is, the tissue contact affects theimpedance measurements of the electrodes.

The tissue contact model shown in FIG. 16A may be further expressed as asimplified electrical circuit 162, as shown in FIG. 16B. For drivesignal frequencies utilized for proximity or contact assessment,capacitance and resistance at the blood-tissue interface dominateimpedance measurements. Accordingly, capacitive-resistive effects at theblood-tissue interface may be represented in circuit 162 by an exemplaryresistor-capacitor (R-C) circuit 166. The exemplary R-C circuit 166 mayinclude a resistor 168 representing the resistive effects of blood onimpedance, in parallel with a resistor 170 and capacitor 172representing the resistive and capacitive effects of the target tissue92 on impedance. When an electrode 20 ₁ or 20 ₂ has no or little contactwith the target tissue 92, resistive effects of the blood affect the R-Ccircuit 166, and hence also affect the impedance measurements. As theelectrode 20 ₁ or 20 ₂ is moved into contact with the target tissue 92,however, the resistive and capacitive effects of the target tissue 92also affect the R-C circuit 166, and hence also affect the impedancemeasurements.

The effects of resistance and capacitance on impedance measurements maybe better understood with reference to a definition of impedance.Impedance (Z) may be expressed as:Z=R+jX  (7)

where:

R is resistance from the blood and/or tissue;

j an imaginary number indicating the term has a phase angle of +90degrees; and

X is reactance from both capacitance and inductance.

It is observed from the above equation that the magnitude of thereactance component responds to both resistive and capacitive effects ofthe circuit 162. This variation corresponds to the level of contact atthe electrode-tissue interface, and therefore may be used to assess theelectrode-tissue coupling. By way of example, when an electrode isoperated is primarily in contact with the blood, the impedance islargely resistive, with a small reactive (X) contribution. When theelectrode contacts the target tissue the magnitudes of both theresistive and reactive components increase.

Alternatively, proximity or contact conditions may be determined basedon phase angle. Indeed, determining proximity or contact conditionsbased on the phase angle may be preferred in some applications becausethe phase angle is represented as a trigonometric ratio betweenreactance and resistance. In an exemplary embodiment, the phase anglemay be determined from the impedance measurements. That is, impedancemay be expressed as:Z=|Z|∠ϕ  (8)

where:

|Z| is the magnitude of the impedance; and

ϕ is the phase angle.

The phase angle also corresponds to the level of proximity or contact atthe electrode-tissue interface, and therefore may be used to assess theelectrode-tissue proximity or contact.

While impedance values may be utilized to assess electrode tissuecontact or proximity, such assessments are typically based on anobserved change in an impedance value of an electrode. That is, ameasured impedance of an electrode is typically compared to a benchmarkor baseline impedance value for that electrode. Accordingly, one knownbaselining procedure is to measure an initial impedance of an electrodein a blood pool and utilize this initial impedance as a baseline valuefor subsequent contact determination/comparison. Often, such abaselining or calibration procedure is performed in-vivo at thebeginning of a procedure. That is, actual baseline values (e.g.,empirical values) are often measured as opposed to relying onpredetermined baseline data. However, predetermined (e.g., stored)baseline values may be used. One benefit of performing an in-vivocalibration is that such a procedure accounts for differences that mayexist between patients and/or the physical configuration of a specificsystem. That is, impedance measurements are affected by cathetercabling, electrode size, and any electrode imperfections (e.g.,R_(other) 173; FIG. 16B). Because of this, in-vivo ‘baselining’ or‘calibration’ procedures are most commonly used to establish baselineimpedances for the electrodes such that subsequent impedance changes canbe identified.

Once a baseline impedance value is established for an electrode,subsequent impedance changes for that electrode may be utilized toassess a level of tissue contact and/or tissue proximity. FIGS. 17A-17Cillustrate exemplary levels of tissue contact or proximity between anelectrode 20 of a medical device 16 and patient tissue 92 that may bedetermined from a change in impedance. Exemplary levels of contact orproximity may include “little or no contact” as illustrated by thecontact condition illustrated in FIG. 17A, “light to medium contact” asillustrated by contact condition illustrated in FIG. 17B, and “hardcontact” as illustrated in FIG. 17C. In an exemplary embodiment, theassessment may be output to, for example, the display 68 as shown inFIG. 7 . A contact condition of little or no contact may be experiencedbefore the electrode 20 of the medical device 16 comes into contact withthe target tissue 92. Insufficient contact may inhibit or even preventadequate lesions from being formed when the medical device 16 isoperated to apply ablative energy. A contact condition of light tomedium contact may be experienced when, for example, the electrode 20 ofthe medical device 16 contacts the tissue and slightly depresses thetissue. A contact condition of hard or excessive contact may result inan electrode 20 being depressed deeply into the tissue 92 which mayresult in the formation of lesions which are too deep and/or thedestruction of tissue surrounding the target tissue 92. Accordingly, theuser may desire a contact condition of light to medium contact.

It is noted that the exemplary proximity or contact conditions of FIGS.17A-17C are shown for purposes of illustration and are not intended tobe limiting. Other proximity or contact conditions (e.g., finergranularity between contact conditions) may also exist and/or be desiredby the user. The definition of such proximity or contact conditions maydepend at least to some extent on operating conditions, such as, thetype of target tissue, desired depth of the ablation lesion, andoperating frequency of the ablation energy, to name only a few examples.

The tissue assessment module 116 of the ECU (see FIG. 7 ) may monitorelectrode impedance changes (e.g., for each electrode) relative toestablish baseline impedance values to generate outputs indicative oftissue proximity or contract for each electrode. That is, the tissueassessment module 116 may categorize each electrode as: 1) insufficientelectrode coupling; 2) sufficient electrode coupling; and 3) elevated orexcessive electrode coupling. One embodiment equates the followingchange in impedance values relative to a baseline impedance values forthe noted conditions:insufficient electrode coupling: ΔZ<20sufficient electrode coupling: 20<ΔZ<200elevated/excessive electrode coupling: ΔZ>200

In such an exemplary embodiment, the contact assessment module 116 maybe operatively associated with the processor 96 memory 94 and/ormeasurement circuit 130 to analyze the change in impedance. By way ofexample, upon determining a change in an impedance measurement for anelectrode, the contact assessment module 116 may determine acorresponding proximity of contact coupling condition for the electrodeof the medical device based on the identified change. In an exemplaryembodiment, proximity or contact conditions corresponding to changes inimpedance values may be predetermined, e.g., during testing for any of awide range of tissue types and at various frequencies. The proximity orcontact conditions may be stored in memory 94, e.g., as tables or othersuitable data structures. The processor 96 or contact assessment module116 may then access the tables in memory 94 and determine a proximity orcontact condition corresponding to change in impedance. It is noted thatthe exemplary proximity or contact ranges shown above are shown forpurposes of illustration and are not intended to be limiting. Othervalues or ranges may also exist and/or be desired by the user. Further,and as is more fully discussed herein, different components of impedance(e.g., resistive component, reactive component and/or phase angle) maybe utilized to assess proximity or contact.

If a baselining or calibration procedure is performed for a medicaldevice or catheter having a single electrode, a limited number ofelectrodes and/or a single axis configuration (e.g., See FIG. 2 ), atechnician can simply position the electrodes of the device in the bloodpool away from tissue, and record the baseline impedances. However, whenattempting to acquire baseline impedance information for medical deviceshaving numerous electrodes, large sizes and/or expandable shapes, uniquedifficulties arise. That is, when measuring impedance on high countelectrode catheters and/or on large catheters, it is often impracticalto position the catheter in such a way as to avoid tissue contact on allelectrodes. By way of example, FIG. 18 illustrates an expandable medicaldevice 16 disposed within the right atrium of a heart 92 of a patient.Initially, the medical device 16 is guided to the right atrium using anintroducer 8 routed through an artery of the patient. Once the end ofthe introducer 8 is positioned proximate to the intersection of theartery and atrium, the expandable basket medical device 16 is disposedthrough the open end of the introducer 8 and into the atrium. At thistime, the shape metal arms 18 a-d (hereafter 18 unless specificallyreferenced) of the medical device 16 expand to the illustrated shape. Asdiscussed above, such a medical device 16 may include multipleelectrodes or pairs of electrodes on each arm 18. Due to the size of themedical device 16 and the limited volume of the atrium, it typically isnot feasible to maneuver the medical device 16 into a position where allelectrodes of the different arms 18 are concurrently disposed within aninterior volume of the atrium (e.g., into a blood pool) and free ofcontact with patient tissue (e.g., atrium wall/surface). That is, if afirst arm 18 c of the medical device 16 is within the blood pool, anopposing arm 18 a will typically be in contact with tissue. Accordingly,a different method for establishing baselines impedance values for eachelectrode of high electrode count catheters is needed.

A first process 360 for establishing baseline impedance values formultiple electrodes of a medical device such as a catheter isillustrated in FIG. 19 . Generally, the process entails identifying aminimum impedance value for each electrode over a time period/window andutilizing the identified minimum impedance value as a baseline impedancevalue for that electrode. Initially, a medical device is positioned 362within an internal patient cavity (e.g., heart chamber). Once initiallypositioned, at least a portion of the electrodes of the medical deviceare expected to be positioned within the interior of the internalpatient cavity. For example, at least a portion of the electrodes arepositioned within a blood pool of the cavity and free from contact withpatient tissue. Once initially positioned, drive signals are applied 364(e.g., continuously applied) to the electrodes of the medical device.Impedance values are measured for each electrode in response to thedrive signals and the impedance values are recorded 366 as initialbaseline impedance values for each electrode. Once an initial set ofbaseline impedance values are recorded, the medical device may be moved368 within the internal patient cavity in conjunction with applicationof the drive signals to the electrodes. Such movement may allow aportion of the electrodes to be repositioned such that they are locatedwithin the blood pool and away from patient tissue. Conversely, aportion of the electrodes may be positioned from within the blood pooland into contact with patient tissue. Subsequent impedance values (e.g.,current impedance values) are measured 370 for each electrode. Thesubsequent impedance value for each electrode is then compared 372 withthe baseline impedance value for that electrode. If the subsequentimpedance value is less than the baseline impedance value for anelectrode 374, the baseline impedance value is reset 376 to thesubsequent impedance value. If the subsequent impedance value is greaterthan the baseline impedance value, the subsequent impedance value may bediscarded. After comparisons are made, the process 360 may continue withadditional catheter movement 368 and measurement 370 of subsequentimpedance values. In this regard, a series of subsequent impedancevalues (e.g., time series of impedance values) may be measured for eachelectrode and each of the series of subsequent impedance values may becompared with previously established baseline value. The process maycontinue until no subsequent impedance value is less than a previouslymeasured baseline impedance value. Alternatively, the process maycontinue for a predetermined time (e.g., time window). In the latterregard, such a time window may have a duration during which eachelectrode would be expected to be disposed within a blood pool for atleast one impedance measurement while the medical device is moved aboutthe patient cavity. Once final baseline impedance values are establishedfor the electrodes, subsequent impedance measurements may be comparedwith the final baseline impedance values to generate indication oftissue proximity or contact.

Another process 380 for establishing baseline impedance values formultiple electrodes of a medical device is illustrated in FIG. 20 .Generally, the process entails identifying a minimum impedance value foreach electrode during a medical procedure (e.g., ablation procedure) andutilizing the identified minimum impedance value as a baseline impedancevalue for determining tissue proximity or contact. Initially, a set ofbaseline impedance values may be established 382 for each electrode. Thebaseline impedance values may be established as set forth above or thesevalues may be established based on predetermined or default values.During operation of the medical device, drive signals are applied 384 toelectrodes of the medical device. In response to the drive signals beingapplied to the electrodes, an impedance value (e.g., current impedancevalue) is measured 386 for each electrode. The current impedance valuesmay be stored. A comparison is made between the current impedance valueand the baseline impedance value for each electrode to determine 388 ifthe current impedance value is less than the baseline impedance value.If the current impedance value is greater than the baseline impedancevalue, a tissue proximity or contact indication is generated 390 basedon, for example, a difference in these impedance values. The tissueproximity or contact indication may be stored and/or output to a user.If the current impedance value is less than the baseline impedancevalue, the baseline impedance value is reset 392 to the currentimpedance value. That is, a new minimum impedance value for electrode(e.g., current impedance value) replaces a previous baseline impedancevalue for that electrode. Optionally, previous tissue proximity orcontact indications for the electrode may be updated 396 based on thenew baseline value. For example, where tissue proximity or contactindications are based on a difference between a baseline impedance valueand a current impedance value, resetting of the baseline impedance valueto a new lower impedance value may result in changes to previouslydetermined tissue proximity or contact indications. Accordingly, anyupdates to the previously determined tissue proximity or contactindications may be stored and/or be output to a user. The process maycontinue for the duration of the procedure.

Though discussed above in relation to establishing baseline impedancevalues (e.g., minimum impedance values) on an electrode-by-electrodebasis, the present disclosure provides another process for establishingbaseline impedance values. Specifically, a process is provided forestablishing spatially-dependent baseline impedance values. Along theselines, baseline impedance values may be established for differentregions of an internal patient cavity. FIG. 21 illustrates a medicaldevice 16 as disposed within the right atrium of a patient heart. Theinterior of the right atrium (e.g., internal patient cavity) is disposedwithin a three-dimensional space that may be divided into sub-regions asillustrated by the grid 78 shown in FIG. 21 . Though shown as atwo-dimensional grid for purposes of illustration, it will beappreciated that such a grid 78 may be three dimensional and that thesize of its grid cells may be modified. Further, sub-regions (e.g., gridcells) may be otherwise defined (radius-based, region based, designatedby an operator, etc.). As noted above, the ECU 72 illustrated in FIG. 7may be configured to obtain data from various external patch electrodesas well as electrodes of a medical device/catheter to determine thelocation of the catheter electrodes within a patient (e.g., within thethree-dimensional space). As previously discussed, the position of anelectrode or multiple electrodes may be determined by driving currentbetween different sets of patches and measuring one or more impedancesor other electrical responses. The ability to locate the position ofeach electrode within a three-dimensional space in conjunction withobtaining impedance values for the electrodes allows for determiningimpedances at locations (e.g., sub-regions) within the three-dimensionalspace. Accordingly, the impedances identified for each sub-region withinthe three-dimensional space may be utilized as baseline impedance valuesfor tissue proximity or contact assessment and/or for lesion formationassessment.

FIG. 22 illustrates a process 400 for establishing spatially-dependentbaseline impedance values. Initially, the process entails identifying402 spatial locations of electrodes of a medical device disposed withinan internal patient cavity (e.g., heart chamber) 402. In conjunctionwith the identification of the location of the electrodes, the processincludes measuring impedance values for each electrode and assigning 404the impedance values to the location of their corresponding electrode.For instance, each electrode may be identified within one sub-region(e.g., grid cell) of a three-dimensional space and each sub-regionincluding an electrode may be assigned the impedance value of theelectrode located therein. Sub-regions that do not include an electrodemay be assigned a null value or a default value. The medical device isrepositioned or moved 406 within the cavity to relocate the electrodes.In conjunction with such movement, the process further includesidentifying 408 updated locations and updated impedance values for theelectrodes. That is, the current locations and current impedance valuesare obtained for the electrodes. Once impedance values are updated, adetermination 410 is made based on the location of each of theelectrodes. Specifically, it is determined if a baseline impedancevalues exist for the current locations of the electrodes. For eachlocation having an existing impedance baseline value (e.g., previouslyassigned baseline impedance value) the current impedance value for thatlocation is compared 412 with the previously assigned baseline impedancevalue for that location. If the current impedance value is less than thebaseline impedance value for that location, the baseline impedance valueis reset 414 to the current impedance value. Each location that does nothave an existing impedance baseline value is assigned 416 the currentimpedance value for that location. This process may continue until nocurrent impedance values are less than a previously measured baselineimpedance value for any location. Alternatively, the process maycontinue for a predetermined time (e.g., time window).

The spatially-dependent baseline impedance values may be used forassessing proximity or contact between an electrode and patient tissue.In addition, spatially-dependent baseline impedance values may providean improved means for assessing lesion formation in tissue. That is,during a procedure where baseline impedance values are assessed forsub-regions that correspond to tissue surfaces (e.g., wall of theinternal patient cavity), changes in subsequent impedance valuescorrespond with changes in the tissue itself. Thus, these changesprovide an indication of lesion formation.

After or in conjunction with establishing baseline impedance values,subsequent impedance measurements may be compared to the to the baselineimpedance values to generate an indication of tissue proximity orcontact. This may be done in various ways. In the simplest form, achange between a subsequent impedance value and a baseline impedancevalue, greater than predetermined threshold value (which may be setheuristically or empirically), indicates tissue contact. That is, whenthe impedance change is greater that the threshold value, tissue contactis considered to exist between an electrode and the tissue. Such asimplified tissue contact assessment may be sufficient when a binaryindication of tissue contact is all that is required. For instance, suchbinary tissue contact may be utilized to map the interior of a patientcavity. Such a binary contact assessment for mapping is shown in FIGS.23A and 23B, which illustrate surfaces (e.g., voltage maps) generatedbased on contacts between electrodes and patient tissue within aninternal patient cavity. As shown, the maps plot identified locations ona display that correspond to physical locations of the electrodes withinthe patient. FIG. 23A illustrates a display output generated by amedical device utilized to map the right atrium of a patient heart.During such a procedure, after establishing baseline impedance values orin conjunction with establishing baseline impedance values, the medicaldevice is moved around the atrium to map its interior space. Each timean electrode contacts tissue and a resulting change of the measuredimpedance differs from a baseline impedance for that electrode by morethan the threshold amount, a location of the contacting electrode isrecorded and output on the display. Over time, a surface geometry forthe interior of the cavity (e.g., atrium) may be generated. FIG. 23Billustrates mapping of the same space where a threshold for binarycontact assessment is reduced. As shown, by reducing the threshold, morecontact points may be identified resulting in a more complete map of theinterior of the patient cavity. As all of the impedance values arestored, a user may perform a mapping procedure and then adjust thethresholds to correspondingly the contact points shown on the map.Further, maps having different thresholds may be combined to generatecomposite maps.

In other applications, multiple thresholds may be utilized. Suchmultiple thresholds may allow generating indicators associated withvarious levels of tissue contact. For example, instead of using binarythresholds, the impedance values may be utilized as indicators of tissuecontact confidence. For instance, ranges of contact indications (e.g.,insufficient, sufficient, elevated, etc.) may be generated as set forthabove. When output to a display, such indication could, for example, beused to color geometry surfaces, scale voltage maps, and/or assist withlesion prediction.

In addition to utilizing the measured impedance values to assess tissueproximity or contact, it will be noted that different components of theimpedance values may be utilized for such assessments. That is, thesystems and processes discussed above measure both resistive (real) andreactive (quadrature) impedance. When assessing tissue contact, thecorrelation between these two components is very high, with theresistive component changing the most. That is, the phase of theimpedance is constant, and changes to the phase are minimal. Thequadrature component (and phase), however, changes significantly when anelectrode enters and exits an introducer (i.e., is sheathed orunsheathed) and when an electrode that comes in contact with anotherelectrode. This is illustrated in FIG. 24 , which shows the quadratureimpedance response of one electrode prior to tissue contact 180, duringtissue contact 182, after tissue contact 184, and during sheathing orcontact with another electrode 186 (or other anomalous event). As shown,the change in the quadrature component is minimal before, during andafter tissue contact. In contrast, when the electrode is sheathed orcontacts another electrode, the quadrature response of the electrodespikes. Accordingly, in one embodiment, it may be sufficient to utilizethe resistive component of impedance (e.g., real component) for tissuecontact assessment and to utilize the quadrature component of impedanceto differentiate between an increase of impedance due to tissue contactversus an increase of impedance due to sheathing and/or contacting otherelectrodes. This may serve an important purpose if a medical device isbeing used autonomously during a procedure to assess and/or excludeother data. In the latter regard, increases of the quadrature componentof the impedance above a predetermined threshold may indicate sheathingand/or contact with other electrode(s) and any impedance measurement(e.g., resistive measurements) obtained during this time may bediscarded. That is, the impedance measurements may be invalidated due tosheathing or electrode contact interference. Alternatively, themagnitude/amplitude of any drive signals and/or any ablation energyapplied to applied to any electrode identified as having an increasedquadrature component of impedance may be altered (e.g., reduced).

In an embodiment, the magnitude of the quadrature component may beutilized to identify when an electrode enters or exits a sheath. Asillustrated in FIG. 24 , spikes in the quadrature component (e.g.,section 186) may indicate a number of anomalous events including contactwith another electrode, an electrode entering or exiting an introducerand/or the failure of a path electrode (e.g., see FIG. 7 ). By way ofexample only, different events may be associated with differentthresholds. Referring again to FIG. 24 , it is noted that the magnitudeof the quadrature component is set forth in Ohms. The magnitude of adeviation of the magnitude of the quadrature component may be correlatedto different event, for example, empirically. Small deviations (e.g.,10-15 Ohms) may represent contact with other electrodes. Largermagnitude deviations (e.g., 20-50 Ohms) may be identified as entry orexit of an electrode into or from an introduce. High magnitudedeviations (e.g., 200+ Ohms) may be identified as patch failure (e.g., abody patch disconnect). It will be appreciated that the ranges discussedare exemplary and other ranges may be established and/or correlated toother events.

While contact assessment and/or lesion assessment is facilitated usingthe above-noted systems and processes, it recognized that electrodeimpedance measurements are often not a stable quantity due to patientrespiration and/or cardiac motion of the heart. Along these lines, thereare instances that an electrode may be in a blood pool but come intoperiodic contact with tissue due to generally period respiration and/orcardiac motion. FIGS. 25 and 26 illustrates two graphs 210 and 230 thatillustrate the impedance response of a plurality of electrodes overtime. More specifically, FIG. 25 illustrates the response of a medicaldevice similar to the device shown in FIG. 17C where the device has fourarms (e.g., splines) and each arm has four electrodes. In the responsegraph 210 illustrated in FIG. 25 , a single arm of the medical deviceand its four electrodes are in contact with patient tissue while theremaining electrodes (e.g., attached to other arms) are out of contactwith the patient tissue. As shown, the magnitude of the responses 212,214, 216 and 218 (shown in Ohms) of the four contacting electrodes varyover time while the responses (collectively 220) of the non-contactingelectrodes remain substantially constant over time. The variation of theresponses 212-218 for the contacting electrode are better illustrated inFIG. 26 , which illustrates these responses before, during and afterablation.

Referring to response 212 in FIG. 26 , it is noted that the response 212has periodic major peak magnitudes 232 a-232 n that correspond withrespiration of the patient. The response 212 also has minor peakmagnitudes 234 a-234 n that correspond with cardiac motion. When contactexists and is maintained between electrodes and tissue, it is observablethat respiration and/or cardiac motion often (but not always) modulatesthe level of contact (e.g., impedance magnitude). Along these lines, acontact assessment scheme utilizing a single contact threshold mayintermittently trigger back and forth due to respiration and/or cardiacmotion. This may be perfectly acceptable, as it accurately portrays amodulating level of contact. However, it may also be a nuisance for auser as the contact levels continually fluctuate. To counteract suchcontinual fluctuation, contact assessment measurements may be timeaveraged or obtained during common times during a respiratory or cardiaccycle. In the former regard, impedance measurements may be acquired andaveraged over a time window that corresponds with, for example, a singleheartbeat or respiration cycle. Such a time window 240 is illustrated inconjunction with response 218 where the horizontal line represents anaverage impedance 242 over the time window 240. In the latter regard,impedance measurements may be taken at common points or phases (e.g.,peak or trough points) of a respiration or cardiac cycle. That is,impedance measurements may be correlated with respiratory and/or cardiacmotion. In such instances, the system (e.g., ECU) may obtain respiratoryor cardiac motion information from one or more sensors (e.g., EGC lead90; see FIG. 7 ). Such a time windowing approach or correlation approachmay improve the stability of contact assessment outputs. However, thismay result in a modest delay between contact updates. Further, the timewindowing approach and correlation approach may each be utilized inestablishing baseline impedance values.

FIG. 26 further illustrates impedance monitoring throughout a medicalprocedure. Specifically, FIG. 26 illustrates continual impedancemonitoring of four electrodes in contact with patient tissue before,during and after application of ablation energy to the tissue. As shown,the responses 212-218 change between pre-ablation contact and duringablation. In the illustrated graph 230, the impedance responses 212-218have a reduced magnitude during ablation compared to pre-ablation.Further, the magnitude of the responses 212-218 tend to slightlyincrease post-ablation. However, it is noted that the post-ablationmagnitudes of the impedance responses do not return to pre-ablationmagnitudes. Referring to impedance response 212, it is noted that a peakmagnitude 232 n of impedance response 212 prior to ablation is greaterthan a peak magnitude 232 z after ablation. This difference Δ providesan indication of lesion formation in the tissue. Accordingly, such adifference may be utilized by the processor or contact assessment moduleto generate an output indicative of lesion formation in tissue. Further,this indication of lesion formation may be incorporated onto or into,for example, the maps shown in FIGS. 23A and 23B. That is, the locationof an electrode and the change in impedance before and after ablationmay be plotted onto a map. Along these lines, the ability to establishspatially-dependent baseline impedance values as discussed in relationto FIG. 22 provides a mechanism for readily recording impedance changeson a surface of the patient tissue in the event that the electrodes moveduring an ablation procedure. For instance, if an ablation electrode isdrawn over tissue during application of ablation energy to create alinear lesion, the location of the electrode providing an impedancemeasurement for a tissue location may change. However, when utilizingmedical devices having high numbers of electrodes (e.g., 100 or 200), itis expected that another electrode will measure the response of thetissue location such that pre-ablation and post-ablation impedances maybe compared for the tissue location.

Various embodiments are described herein to various apparatuses,systems, and/or methods. Numerous specific details are set forth toprovide a thorough understanding of the overall structure, function,manufacture, and use of the embodiments as described in thespecification and illustrated in the accompanying drawings. It will beunderstood by those skilled in the art, however, that the embodimentsmay be practiced without such specific details. In other instances,well-known operations, components, and elements have not been describedin detail so as not to obscure the embodiments described in thespecification. Those of ordinary skill in the art will understand thatthe embodiments described and illustrated herein are non-limitingexamples, and thus it can be appreciated that the specific structuraland functional details disclosed herein may be representative and do notnecessarily limit the scope of the embodiments, the scope of which isdefined solely by the appended claims.

Reference throughout the specification to “various embodiments,” “someembodiments,” “one embodiment,” or “an embodiment”, or the like, meansthat a particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodiment.Thus, appearances of the phrases “in various embodiments,” “in someembodiments,” “in one embodiment,” or “in an embodiment”, or the like,in places throughout the specification are not necessarily all referringto the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments. Thus, the particular features, structures, orcharacteristics illustrated or described in connection with oneembodiment may be combined, in whole or in part, with the featuresstructures, or characteristics of one or more other embodiments withoutlimitation given that such combination is not illogical ornon-functional.

Although numerous embodiments have been described above with a certaindegree of particularity, those skilled in the art could make numerousalterations to the disclosed embodiments without departing from thespirit or scope of this disclosure. For instance, the present disclosurediscusses a bi-pole configuration where each pair of electrodes isindependent of all other pairs of electrodes. However, anotherpossibility is to configure electrodes such that one side of eachbi-pole is a common electrode. For example, with reference to thecatheter of FIG. 2 , the tip electrode 22 may form a common electrodefor each of the additional ring electrodes 20A-I. That is, tip electrode22 may be one electrode of each pair of electrodes.

All directional references (e.g., plus, minus, upper, lower, upward,downward, left, right, leftward, rightward, top, bottom, above, below,vertical, horizontal, clockwise, and counterclockwise) are only used foridentification purposes to aid the reader's understanding of the presentdisclosure, and do not create limitations, particularly as to theposition, orientation, or use of the any aspect of the disclosure. Asused herein, the phrased “configured to,” “configured for,” and similarphrases indicate that the subject device, apparatus, or system isdesigned and/or constructed (e.g., through appropriate hardware,software, and/or components) to fulfill one or more specific objectpurposes, not that the subject device, apparatus, or system is merelycapable of performing the object purpose. Joinder references (e.g.,attached, coupled, connected, and the like) are to be construed broadlyand may include intermediate members between a connection of elementsand relative movement between elements. As such, joinder references donot necessarily infer that two elements are directly connected and infixed relation to each other. It is intended that all matter containedin the above description or shown in the accompanying drawings shall beinterpreted as illustrative only and not limiting. Changes in detail orstructure may be made without departing from the spirit of the inventionas defined in the appended claims.

Any patent, publication, or other disclosure material, in whole or inpart, that is said to be incorporated by reference herein isincorporated herein only to the extent that the incorporated materialsdoes not conflict with existing definitions, statements, or otherdisclosure material set forth in this disclosure. As such, and to theextent necessary, the disclosure as explicitly set forth hereinsupersedes any conflicting material incorporated herein by reference.Any material, or portion thereof, that is said to be incorporated byreference herein, but which conflicts with existing definitions,statements, or other disclosure material set forth herein will only beincorporated to the extent that no conflict arises between thatincorporated material and the existing disclosure material.

What is claimed is:
 1. A method for use with a medical device configuredfor insertion within a patient, comprising: applying a plurality ofdrive signals, each drive signal having a unique frequency that is aharmonic of a common base frequency, across different pairs ofelectrodes of a medical device having a plurality of electrodes;measuring a series of subsequent impedance values for each of theplurality of electrodes of the medical device in response to theapplying of the plurality of drive signals; and for each electrode:comparing each subsequent impedance value of the series of subsequentimpedance values to a baseline impedance value for the electrode;generating an indication of tissue proximity of the electrode relativeto tissue if the subsequent impedance value is greater than the baselineimpedance value; and displaying the indication of tissue proximity ofthe electrode relative to the tissue on a display.
 2. The method ofclaim 1, further comprising: resetting the baseline impedance value tothe impedance value if the subsequent impedance value is less than thebaseline impedance value.
 3. The method of claim 1, wherein measuringthe series of subsequent impedance values further comprises:synchronously demodulating responses of the plurality of electrodes tothe simultaneously applied drive signals for each of the uniquefrequencies.
 4. The method of claim 1, wherein the measuring of theseries of subsequent impedance values for each of the plurality ofelectrodes further comprises: measuring the subsequent impedance valuefor each of the plurality of electrodes during movement of the medicaldevice, wherein positions of at least a portion of the plurality ofelectrodes changes.
 5. The method of claim 1, wherein the indication oftissue proximity comprises one of: a binary indication of contact ornon-contact between the electrode and the tissue; and a range of contactconditions indicative of a level of contact between the electrode andthe tissue.
 6. The method of claim 1, wherein the impedance valuescomprise complex impedance values having a real component and aquadrature component.
 7. The method of claim 6, wherein the comparingand the generating further comprise: comparing the real component of theimpedance value to a real component of the baseline impedance value; andgenerating the indication of tissue proximity based on the comparison ofthe real component of the impedance value and a real component of thebaseline impedance value.
 8. The method of claim 6, further comprising:comparing the quadrature component of the impedance value to aquadrature component of the baseline impedance value; and upon thequadrature component of the impedance value exceeding the quadraturecomponent of the baseline impedance value by a predetermined threshold,generating an output.
 9. The method of claim 6, wherein generating theoutput comprises generating at least one of: an indication that anelectrode has entered or exited an introducer; and an indication that apatch electrode used in the applying of the plurality of drive signalsis malfunctioning.
 10. The method of claim 1, further comprising, foreach electrode: identifying a location of the electrode relative to aninternal patient cavity, wherein the indication of tissue proximity isdisplayed on the display at a corresponding location on a map of theinternal patient cavity.
 11. The method of claim 1, further comprising:receiving an input indicative of motion of the tissue caused by at leastone of respiration and cardiac motion, wherein the indication of tissueproximity is altered based on the indication of motion.
 12. The methodof claim 11, wherein the current impedance values are generated incorrelation with a phase of the motion.
 13. A non-transitory computerreadable medium storing computer executable instructions, executable bya processor to: apply a plurality of drive signals, each drive signalhaving a unique frequency that is a harmonic of a common base frequency,across different pairs of electrodes of a medical device having aplurality of electrodes; measure a series of subsequent impedance valuesfor each of the plurality of electrodes of the medical device inresponse to the applying of the plurality of drive signals; and for eachelectrode: compare each subsequent impedance value of the series ofsubsequent impedance values to a baseline impedance value for theelectrode; generate an indication of tissue proximity of the electroderelative to tissue if the subsequent impedance value is greater than thebaseline impedance value; and display the indication of tissue proximityof the electrode relative to the tissue on a display.
 14. Thenon-transitory computer readable medium of claim 13, further comprisinginstructions executable by the processor to: reset the baselineimpedance value to the impedance value if the subsequent impedance valueis less than the baseline impedance value.
 15. The non-transitorycomputer readable medium of claim 13, further comprising instructionsexecutable by the processor to: measure the subsequent impedance valuefor each of the plurality of electrodes during movement of the medicaldevice, wherein positions of at least a portion of the plurality ofelectrodes changes.
 16. The non-transitory computer readable medium ofclaim 13, further comprising instructions executable by the processor togenerate at least one of: a binary indication of contact or non-contactbetween the electrode and the tissue; and a range of contact conditionsindicative of a level of contact between the electrode and the tissue.17. The non-transitory computer readable medium of claim 13, wherein theimpedance values comprise complex impedance values having a realcomponent and a quadrature component.
 18. The non-transitory computerreadable medium of claim 17, further comprising instructions executableby the processor to: compare the real component of the impedance valueto a real component of the baseline impedance value; and generate theindication of tissue proximity based on the comparison of the realcomponent of the impedance value and a real component of the baselineimpedance value.
 19. The non-transitory computer readable medium ofclaim 17, wherein the step of generating the output comprises generatingat least one of: an indication that an electrode has entered or exitedan introducer; and an indication that a patch electrode used in theapplying of the plurality of drive signals is malfunctioning.
 20. Thenon-transitory computer readable medium of claim 13, further comprisinginstructions executable by the processor to: identify a location of theelectrode relative to an internal patient cavity, wherein the indicationof tissue proximity is displayed on the display at a correspondinglocation on a map of the internal patient cavity.